The alpha-helix constitutes one of the principal architectural features of peptides and proteins. It is a rod-like structure wherein the polypeptide chain coils around like a corkscrew to form the inner part of the rod and the side chains extend outward in a helical array. Approximately 3.6 amino acid residues make up a single turn of an alpha-helix; thus the side chains that are adjacent in space and make up a “side” of an alpha-helix occur every three to four residues along the linear amino acid sequence. The alpha-helix conformation is stabilized by steric interactions along the backbone as well as hydrogen bonding interactions between the backbone amide carbonyls and NH groups of each amino acid. Nearly a third of the residues in known proteins form alpha-helices and such helices are important structural elements in various biological recognition events, including ligand-receptor interactions, protein-DNA interactions, protein-RNA interactions, and protein-membrane interactions. Given the importance of alpha-helices in biological systems, it would be desirable to have available small organic molecules that act as mimics of alpha-helices. Such compounds would be useful not only as research tools, but as therapeutics to treat conditions mediated by alpha-helix binding enzymes and receptors. Yet, despite the wealth of research on other aspects of alpha-helices, relatively little research has been devoted to identifying small molecule alpha-helix mimetics and there remains a need in the art for such compounds.
Galanin is a peptide hormone of diverse biological effect found through out the nervous and endocrine systems of a number of species. It binds to at least three different G-protein coupled receptors (GalR1-3) and influences such processes as insulin secretion, gut secretion/motility, memory, sexual behavior, and pain regulation among others. Site-directed mutagenesis studies on a sixteen-amino acid fragment have shown that this peptide binds to galanin receptor type 1 (GalR1) through three amino acid residues (Trp2, Asn5, Tyr9), thought to be in an alpha-helical conformation, as well as through the N-terminal residue. It is desirable to find low molecular weight molecules that function as potent agonists of GalR1 and possess extended in vivo stability; such compounds would be potential new analgesics. Similarly, agonists of GalR2 receptors may find also use in treating pain or dementia in Alzheimer's patients. See Branchek, T.; et al., Ann. NY Acad. Sci. 1998, 863, 94.
Bak and Bcl-xL belong to the Bcl-2 family of proteins, which regulate cell death through an intricate balance of homodimer and heterodimer complexes formed within this class of proteins. [M. C. Raff, Science 1994, 264, 668-669; D. T. Chao, S. J. Korsmeyer, Annu. Rev. Immunol. 1998, 16, 395-419; C. B. Thompson, Science 1995, 267, 1456-1462; L. L. Rubin, K. L. Philpott, S. F. Brooks, Curr. Biol. 1993, 3, 391-394]. Overexpression of anti-apoptotic proteins such as Bcl-xL and Bcl-2 prevent cells from triggering programmed death pathways and has been linked to a variety of cancers. Bcl-2 protein plays a critical role in inhibiting anticancer drug-induced apoptosis, which is mediated by a mitochondria-dependent pathway that controls the release of cytochrome c from mitochondria through anion channels. Constitutive overexpression of Bcl-2 or unchanged expression after treatment with anticancer drugs confers drug resistance not only to hematologic malignancies but also to solid tumors [R. Kim et al. Cancer 2004, 101, 2491-2502]. A current strategy for developing new anticancer agents is to identify molecules that bind to the Bak-recognition site on Bcl-xL, disrupting the complexation of the two proteins and therefore antagonizing Bcl-xL function [O. Kutzki et al. J. Am. Chem. Soc. 2002, 124, 11,832-11,839]. The structure determined by NMR spectroscopy [M. Sattler et al. Science 1997, 275, 983-986] shows the 16 residue BH3 domain peptide from Bak (aa 72 to 87, Kd≈300 nM) bound in a helical conformation to a hydrophobic cleft on the surface of Bcl-xL, formed by the BH1, BH2, and BH3 domains of the protein. The crucial residues for binding were shown by alanine scanning to be V74, L78, I81, and I85, which project in an i, i+4, i+7, i+11 arrangement from one face of the α-helix. The Bak peptide is a random coil in solution but adopts an α-helical conformation when complexed to Bcl-xL. Studies utilizing stabilized helices of the Bak BH3 domain have shown the importance of this conformation for tight binding. [J. W. Chin, A. Schepartz, Angew. Chem. 2001, 113, 3922-3925; Angew. Chem. Int. Ed. 2001, 40, 3806-3809.]
Small molecule mimetics of alpha-helices are of immense pharmaceutical interest and would circumvent the problems associated with the use of peptidic agents. Accordingly, there is a need in the art for small molecule compounds that can modulate the activity of alpha-helix mediated interactions and therefore would be useful in the treatment of a variety of diseases mediated by these proteins.